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Status and potential of atmospheric plasma processing of materialsDaphne Pappas Citation: Journal of Vacuum Science & Technology A 29, 020801 (2011); doi: 10.1116/1.3559547 View online: http://dx.doi.org/10.1116/1.3559547 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/29/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in ZnO deposition on metal substrates: Relating fabrication, morphology, and wettability J. Appl. Phys. 113, 184905 (2013); 10.1063/1.4803553 Radical modification of the wetting behavior of textiles coated with ZnO thin films and nanoparticles whenchanging the ambient pressure in the pulsed laser deposition process J. Appl. Phys. 110, 064321 (2011); 10.1063/1.3639297 Surface texture and wetting stability of polydimethylsiloxane coated with aluminum oxide at low temperature byatomic layer deposition J. Vac. Sci. Technol. A 28, 1330 (2010); 10.1116/1.3488604 Atmospheric oxygen plasma activation of silicon (100) surfaces J. Vac. Sci. Technol. A 28, 476 (2010); 10.1116/1.3374738 Characteristics and anticoagulation behavior of polyethylene terephthalate modified by C 2 H 2 plasmaimmersion ion implantation-deposition J. Vac. Sci. Technol. A 22, 170 (2004); 10.1116/1.1633569

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REVIEW ARTICLE

Status and potential of atmospheric plasma processing of materialsDaphne Pappasa�

United States Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005

�Received 4 February 2011; accepted 4 February 2011; published 4 March 2011�

This paper is a review of the current status and potential of atmospheric plasma technology formaterials processing. The main focus is the recent developments in the area of dielectric barrierdischarges with emphasis in the functionalization of polymers, deposition of organic and inorganiccoatings, and plasma processing of biomaterials. A brief overview of both the equipment being usedand the physicochemical reactions occurring in the gas phase is also presented. Atmospheric plasmatechnology offers major industrial, economic, and environmental advantages over otherconventional processing methods. At the same time there is also tremendous potential for futureresearch and applications involving both the industrial and academic world. © 2011 AmericanVacuum Society. �DOI: 10.1116/1.3559547�

I. INTRODUCTION

A. Low pressure plasma technology

Extensive work related to low-pressure plasma processingof materials has been published since the early 1960s.1 Thesestudies focus on the development and characterization ofvarious plasma sources utilizing inert gases such as He andAr, as well as reactive gases such as O2, H2, CH4, etc.2,3

Literature references present results from the electrical char-acterization of the discharges, while employment of gas-phase diagnostics such as optical emission spectroscopy�OES� and mass spectrometry provides information about theidentity and quantity of excited and charged plasma species,respectively.4,5 Besides the experimental investigation, aplethora of work has presented results from the numericalsimulation of low-pressure discharges, which predict the ef-fect of the plasma parameters �power, interelectrode distance,pressure, and others� on the surface properties of materialsexposed to these environments.6,7

Evaluation of experimental results obtained by differentplasma research groups using various chamber geometrieshas always been a difficult task. To overcome the problem, acollaborative experimental effort was initiated at a workshopat the 1988 Gaseous Electronics Conference �GEC� to start aprogram to understand the fundamental physics of process-ing plasmas.8 The program’s aim was to give researchers abaseline experiment to develop plasma diagnostics to beused on manufacturing plasma systems. Toward that direc-tion, the program required the design of an experimentalchamber based on the use of 4 in. diameter aluminum elec-trodes in a parallel plate configuration at 13.56 MHz, whichwas run in a capacitively coupled discharge mode. Unfortu-nately, this approach was not widely accepted due mainly toeconomic restrictions as the various plasma groups would

have to cease utilization of their existing equipment andspend funds on the purchase of new reactors based on theGEC model. Since then, there has been considerable debateabout the set of operating parameters that need to be con-trolled and the precision that measurements must have inorder to replicate experiments at different laboratories.9

Despite the disadvantages of using a variety of reactorconfigurations, low-pressure plasma technology has contrib-uted to the development of processes that produce advancedmaterials. Of particular interest is the plasma treatment ofmaterials, these being in the form of thin films, sheets, fibers,or woven fabrics. Plasma modification can result in the tai-loring of the surface properties of the polymers to overcomesome of the problems associated with low chemical reactiv-ity and wettability without adversely affecting the bulk prop-erties of the material.

Low pressure plasma methods have been studied for de-cades as a technique to modify polymeric surfaceproperties.10 In this process a material is exposed to an en-vironment of plasma reactive species, such as ions, electrons,excited atoms and molecules, and neutral radicals. Thesespecies cleave existing chemical bonds and form new reac-tive functional groups, which permit grafting, polymeriza-tion, or cross-linking at the surface. Plasma processing canimprove adhesion11 by removing surface contamination andin some cases change surface morphology through increasedroughness due to etching.

B. History of discharges operating under atmosphericpressure

Vacuum processing presents both handling and vacuumequipment cost constraints for high-throughput applications.In recent years increasing interest has been drawn to theapplications of electrical discharges operating under atmo-spheric pressure. As expressed in Paschen’s law �Eq. �1��,a�Electronic mail: [email protected]

020801-1 020801-1J. Vac. Sci. Technol. A 29„2…, Mar/Apr 2011 0734-2101/2011/29„2…/020801/17/$30.00 ©2011 American Vacuum Society


Vb =B�p · d�

ln�A�p · d�� − ln�ln�1 + 1/�se��, �1�

the breakdown voltage �Vb� required to ignite the dischargeis dependent on the process pressure �p� and the gap distance�d� between the electrodes, given that the discharge is con-fined between two electrodes, and ��se� is the secondary ion-ization coefficient. Constants A and B depend on the compo-sition of the gas, their values for air are A=15 cm−1 Torr−1

and B=365 V cm−1 Torr−1.12 Figure 1 shows the breakdownvoltage of parallel plates in a gas as a function of pressureand gap distance.

As depicted in this figure, under atmospheric-pressureconditions the intensity of the electric field needed to causean electrical discharge for a given gap is much greater thanthat required for low-pressure ignition. Also, it can be seenthat by decreasing the interelectrode distance—with normalpressure—the breakdown voltage needed to cause an arc de-creases. For example, at 760 Torr and 5 mm gap, the break-down voltage for Ar is 2500 V. This shows the high-energyrequirements to ignite and sustain an electrical discharge un-der atmospheric pressure.

Although electrical discharges in air have been used forozone generation13 since 1857 by Siemens, their use in otherapplications such as materials processing was limited. Also,atmospheric plasmas have been used as powerful UV, exci-mer lamp sources, and CO2 and N2 atmospheric plasmalasers14,15 for decades. Recently, atmospheric plasma tech-nology has seen significant advancement as systems of sev-eral different geometries are commercially available and areequipped with advanced technology power supplies. Sincethe early work of Kogelschatz, numerous academic and in-dustrial groups in Europe, North America, and Asia havebeen employed in understanding the fundamentals of atmo-spheric plasma processing and its utilization in polymermodification.

One of the most popular discharges operating under atmo-spheric pressure is the dielectric barrier discharge �DBD�.

The setup used for this type of electrical discharge consistsof two metal electrodes with one or both covered by a di-electric material such as AlO3 or quartz. The dielectric layeris present to restrict the high current produced by the appliedvoltage to ignite the discharge. In Siemens’ manuscript16 thistype of electrical discharge was referred to as “silent dis-charge” �stille Entledung in German� and “electrolysis of thegas phase” and was mainly focused on the generation ofozone from air or oxygen. This was achieved by subjecting aflow of oxygen or air to the narrow annular gap between twocoaxial glass tubes. The dielectric barrier discharge was ig-nited and maintained by applying an alternating electric fieldof sufficient amplitude. The novel feature of this dischargeapparatus was that the electrodes were positioned outside thedischarge chamber and were not in contact with the plasma,which is the main difference with contemporary experimen-tal setups, where the electrodes are in direct contact with theplasma.

The next breakthrough in the development of this newtechnology was made by the electrical engineer K. Buss17 inthe 1930s when he discovered that the breakdown of air at760 Torr between planar parallel electrodes covered by di-electrics always occurs in a large number of small-sizedshort-lived current filaments. Buss used photographic film toimage the footprints of the generated microdischarges. DBDsare nonequilibrium filamentary discharges and the appear-ance of microdischarges is a common phenomenon. Moredetails about the generation and operations of DBDs are pre-sented later in the gas-phase reaction section.

C. Atmospheric plasma technology after the 1990s

Until the 1990s atmospheric-pressure discharges weremostly used for excimer laser excitation and ozone produc-tion, where O3 was used for water treatment and sterilization.Although this type of discharges has been known and usedfor several decades, a detailed investigation of their proper-ties and structure was not possible due to technological limi-tations. The need for ultrasensitive image converters and fastcurrent monitoring probes prohibited their detailed study.Also, a new generation of computer systems was necessaryto assist in the numerical simulation studies for calculatingthe electron and ion energy distribution functions and electricfield in the constricted areas where atmospheric plasmas aregenerated, typically within a gap of less than 1 mm. A newera began with the progress in the work of Kogelscaltz, thefundamental studies of He and N2 plasmas by the group ofMassines at the University of Toulouse, the studies of atmo-spheric plasma glow discharges �OAUGDP� by the group ofRoth at the University of Tennessee, the work of Okazakiand Kogoma at the University of Tokyo on the atmospheric-pressure glow discharge �APGD� generation, and the innova-tive plasma medicine work of Professor A. Fridman atDrexel University.

Since the late 1990s the number of published articles hasincreased exponentially. A Web of Science search performedusing “dielectric barrier discharge” as the search termyielded less than 5 articles/year published before 1990, as

10-1 100 101 102 103102

103

104

105airH2N2HeArNe

Voltage(V)

pd (cmTorr)

FIG. 1. �Color online� Breakdown voltage of parallel plates in a gas as afunction of pressure and gap distance for various gases.

Daphne Pappas: Status and potential of atmospheric plasma processing of materials020801-2 020801-2

J. Vac. Sci. Technol. A, Vol. 29, No. 2, Mar/Apr 2011


seen in Fig. 2. In 1997 the number of published articles wasincreased to 68 while in 2009 the number was ten timeshigher. This observation shows the increased interest of thescientific world in the DBD technology and its applications.

This article aims to introduce atmospheric plasma pro-cessing of materials to those having limited knowledge inthis topic and wish to pursue this technology. Also, it is in-tended to briefly present some of the most popular equipmentbeing used and to provide reference to articles published inthe literature. Most importantly, this work concentrates onthe atmospheric-pressure plasma interactions with materialsbeing exposed to such environments and the challenges inovercoming the appearance of microdischarges.

II. EQUIPMENT

Dielectric DBDs are often confused with corona dis-charges. While both can operate under atmospheric-pressureconditions, a corona discharge is produced near sharp tips,thin wires, and rough edges where the electric field is strongenough to initiate breakdown. Therefore, these discharges arealways nonuniform and not suitable for processing of softmaterials.

A. Reactor design

A typical DBD reactor consists of two metal electrodeswith one or both being covered with a dielectric material, asseen in Fig. 3. The interelectrode gap can range between 0.1mm and several centimeters. Most commonly the reactor isplaced in an insulating box to avoid electrical hazards anddirect exposure to the gases used. In cases where specialhousing is present, a mechanical pump can be added to thesystem allowing processing with gases of potential toxicity,such as acetylene and fluorocarbon gases, or to improve thepurity of the reactor environment. In larger scale systems,shields can be placed over the high-voltage electrodes to pro-tect users from electric shocks.

In processes where the treatment of long webs and textilesis involved a cylindrical rotating electrode can be employed�Fig. 4�. These systems are occasionally controlled by a ro-tating motor which enables the upscale of this process to anindustrial roll-to-roll type process, operating at speeds higherthan 5 m/min. Plasma is generated between the lower rotat-ing electrode and either a single or a set of high voltageupper electrodes. A roll of material that needs to be pro-cessed is mounted externally to the reactor and is exposed tothe plasma generated between the electrodes at a high speedrate.

A variation of this setup is an industrial scaleatmospheric-pressure coating system with an integrated liq-uid aerosol precursor delivery system, as shown in Fig. 5.The device consists of two vertical plasma chambers con-taining electrodes housed in a dielectric perimeter.

1940 1950 1960 1970 1980 1990 2000 20100

100

200

300

400

500

600

700

Numberofpublishedarticles

FIG. 2. �Color online� Number of articles published since 1940 showing theincreasing interest of research groups in atmospheric plasma technology.

FIG. 3. �Color online� Configurations of dielectric barrier discharge systems.

FIG. 4. �Color online� Cylindrical dielectric barrier discharge system for thetreatment of films and textiles.

FIG. 5. �Color online� Roll-to-roll dielectric barrier discharge setup for thedeposition of thin coatings using atomized liquid precursors.


JVST A - Vacuum, Surfaces, and Films


The plasma gases used are typically He or a mixture ofHe and O2 �in a flow rate ratio of �1%–10% oxygen inhelium� which are used as carrier gases for the liquid precur-sor, typically organosilicon compounds, and are deliveredinto the plasma via two parallel path pneumatic nebulizersusing a syringe pump. SiOx coatings can be deposited ontopolymer films passing through the plasma at a constant con-trolled speed.18

B. Power supplies and dielectric materials

Frequencies from a few hertz to megahertz are applied togenerate a DBD. The presence of the dielectric material lim-its the operation to ac regimes only. The dielectric layer isincorporated in the system to prevent the formation of arcsand channels of increased current, which are likely to appearunder the application of increased potential, ranging from 0.5to 20 kV. The dielectric material used in the DBD setup alsoplays an important role in sustaining an APGD. Studies re-ported in the literature19 discuss the etching effect of poly-propylene after plasma ignition and without a gas flow,which results in quenching of N2 metastables in a higherdegree compared to the use of Al2O3 as the dielectric. In fact,the maximum voltage value required to maintain an APGDwas 9.5 kV for the setup containing Al2O3 while only 7.5 kVwas the maximum value for the polypropylene dielectric ma-terial. In this case, it is wise to maintain a low applied powerto reduce the etch rate of the dielectric.

The dielectric constant and thickness of the dielectric ma-terial chosen together with the time derivative of the appliedvoltage, dV /dt, determine the amount of displacement cur-rent that can be passed through the dielectric. As mentionedearlier, the dielectric is the key for the proper functioning ofthe discharge. It limits the charge transported in the dis-charge by limiting the current flow to the system and distrib-utes the discharge almost uniformly over the entire electrodearea. In general, glass and ceramics are preferred materialsdue to their low dielectric loss and high breakdown strength.

C. Gas options and flow rates

The gas flow mode is another factor that needs to be takeninto serious consideration when designing atmosphericplasma systems. As described in the previous paragraph,etching products such as O and H can diffuse toward theplasma phase and quench the plasma excited species. DuringDBD processing of polymers low molecular weight frag-ments, resulting from the polymer and dielectric dissociationunder plasma exposure, quench the gas metastable moleculesresponsible for the formation of seed electrons via the Pen-ning effect. One way to overcome this problem is to use ahigh laminar gas flow, around several l/min, instead of ashowerhead gas distributor. That restricts the quenchers to anarea near the electrodes without causing any perturbation tothe bulk plasma phase.19 The gas flow not only decreases thedensity of the gas metastable species but also controls theirspatial distribution in the interelectrode gap. To maintain alaminar flow the Reynolds number, Re—see Eq. �2�—needsto remain below 2000,

Re =� · d · �

�, �2�

where �, d, �, and � stand for the gas velocity, characteristiclength, density, and viscosity, respectively.

In DBD systems helium gas20 is typically used for plasmaignition due to the fact that it is a light gas with high diffu-sion constant and in most cases a uniform filament-free glowdischarge is produced. The main drawback for using He isthe cost associated with its use. To overcome this issue, sev-eral research groups and industries tend to use N2 or air.

Also, the minimum voltage for breakdown depends on thefrequency of the power supply used for plasma ignition. Ac-cording to Gherardi et al. in the 1–15 kHz regime a N2 glowdischarge appears when a 5.5 kV voltage is applied. Severalgroups obtained stable high pressure glow discharges by re-placing the plane electrode with a grid at frequencies as lowas 50 Hz.21

D. Design optimization of DBD systems

The big question still remains: how can nonfilamentaryuniform dielectric barrier discharges be generated? What isthe “ideal” reactor size, electrode shape, spacing betweenelectrodes, frequency of power supply, etc? Obviously, sincethe technology is still at its infancy, the number of manufac-turers offering reliable complete systems is still very smalland most research groups develop their own reactor geom-etries. Therefore, the comparison of results produced bythese systems is a complex task and depends on the systembeing used. However, it is crucial for materials processing toensure that the presence of filaments will not degrade thematerials being exposed to such plasma environments.

The uniformity of DBDs can be improved in two ways:�1� by increasing uniform preionization of the gas and �2� byshortening the voltage rise time.22 A fast rise time of dV /dt�1 kV ns−1 has been claimed as the main factor for dis-charge uniformity.23,24 As an example, when a 13 kV voltageis applied over a 1 mm gap the reduced electric field �E /n� isabout 4�10−15 V cm2 which generates an electron drift ve-locity of 107 cm s−1 and the time required to bridge the 1mm gap is 10 ns.25 This 10 ns time period is critical for thebuildup of local nonuniformities developed in the plasmaphase. Nanosecond power supplies appear to offer promisingfeatures offering less time for nonuniformities to develop,fast expansion of the discharge channels and their overlap-ping, generation of electrons with longer mean free path andgeneration of vacuum ultraviolet �vuv� radiation, and photo-ionization of the gas ahead of the ionization front, as de-scribed in Refs. 26–28. Apparently, it is always helpful tomaintain small interelectrode spacing of 1–1.5 mm, usegases with high diffusivity such as He at high flow rates, andadd small amounts of molecular reactant gases �N2,O2�.

Perhaps it would be a good idea for plasma researchers todevelop a prototype system of optimized geometry that willbe employed in future research and will assist in technologytransfer across different laboratories.




E. Other experimental setups operating underatmospheric-pressure conditions

Plasma jet systems are promising plasma sources for thetreatment of a variety of surfaces. Typically, the plasmasource consists of a thin quartz tube of a few millimeterinternal diameter, which is mounted between two parallelaluminum electrodes. One of the electrodes is supplied withradio frequency power at 13.56 MHz, while the other one isgrounded. Argon, oxygen, and oxygen gas are allowed toflow through the quartz tube, where they were broken downto generate the discharge.29,30 A plume of the plasma glow isallowed to diffuse through the nozzle connected to the tubeand has a diameter that can range from 1 to almost 50 mm.Therefore, the utilization of this system can contribute tosmall scale localized treatments unless it is connected to arobotic arm allowing application over larger areas.

The group of Hana Barankova and Ladislav Bardos fromUppsala University, Sweden has developed hybrid hollowelectrode activated discharge �H-HEAD� sources for the gen-eration of very long columns of cold atmospheric plasma inopen-air arrangement at gas flow rates on the order of 100SCCM �SCCM denotes cubic centimeter per minute at STP�.The source combines microwave plasma with hollow cath-ode plasma using a special tubular antenna electrode termi-nated by a gas nozzle. At the microwave power of 300 W thesource is capable to produce over 70 mm long plasma at lessthan 150 SCCM of air or nitrogen flowing in open air.31

III. ROLE OF ENERGETIC SPECIES INATMOSPHERIC-PRESSURE PLASMAS—GASPHASE CHARACTERIZATION

In order to understand the influence of atmospheric plas-mas on surfaces of materials exposed to them, the gas spe-cies must first be identified and quantified. DBDs are consid-ered nonequilibrium plasmas, which mean that the electronshave a much higher energy, often referred to as “tempera-ture,” than the neutral gas particles. Further, a low degree ofionization is assumed, as the total number density of chargedparticles is much lower than the total number density of theneutral particles. As far as the charged particles are con-cerned, the plasmas studied are quasineutral, with the totaldensity of negative charge carriers being equal to the densityof positive charge carriers. A critical parameter for all non-equilibrium plasmas is the reduced field E /n, where E standsfor the electric field over the neutral gas density n. The unitfor the reduced field is the Townsend �Td�, equal to10−17 V cm2. The reduced field is related to the breakdownstrength of a plasma; for typical gases such as air, oxygen,and nitrogen breakdown occurs at around 100 Td. In mostgases E /n at breakdown corresponds to electron energies ofabout 1–10 eV, which is ideal for the excitation of atomicand molecular species and dissociation of chemical bonds ofmaterials exposed to these plasmas.

Upon the application of a high potential, ranging from 1to 20 kV, breakdown is followed by the formation of a glow.The breakdown voltage depends on the reactor geometry aspredicted by Paschen’s law, shown in Eq. �1�, but typically

larger interelectrode gaps require application of larger poten-tials. As most DBD systems involve the use of two metalelectrodes covered by one or two dielectric layers, a capaci-tive response is expected. Initially, without discharge, thetotal capacitance is charged with rising voltage. Under theapplication of a sinusoidal voltage and after the criticalbreakdown voltage of the gap is achieved, the appearance ofmicrodischarges is observed, having an average diameter of100 �m.

This results in the accumulation of charge on the dielec-tric layer�s� and the reduction of the electric field, while theeffect ceases when the maximum voltage is reached. Duringthe second half of the cycle the phenomenon is reversed andrepeated again later. The microdischarges can often be lo-cated at the same areas observed earlier, exhibiting amemory effect. At a wide range of excitation frequencies, theappearance and density of the microdischarges are dependenton the gas composition and electrode geometry and not onthe electrical parameters chosen for the ignition, as seen inFig. 6.

Figure 6�a� shows a uniform helium discharge producedby our DBD system. The high-voltage electrode is 25 cmlong while plasma can be generated for gaps up to 2.5 cm,allowing the treatment of thicker samples compared to thecapability of other similar systems. Plasma is ignited throughthe application of microsecond pulses. Figure 6�b� shows theeffect of 2% oxygen added to the discharge. The appearanceof microdischarges distributed over the entire length of theelectrode is evident. An increase of the dissipated power oc-casionally results in the generation of a greater number ofmicrodischarges per unit time. Detailed studies of their prop-erties have been performed by several groups.32–34

Simultaneously, the electrical breakdown leads to the ex-citation of some of the neutral atoms or molecules and reac-tion kinetics mechanisms are initiated. The electrons are per-haps the most important species present in the plasmas as

FIG. 6. �Color online� ARL dielectric barrier discharge system, �a� uniformglow and �b� filamentary discharge.




they participate in the excitation processes and have highmobility, and thus their kinetic energy is transferred to theexcited species. The key to optimize the yield of the processis to control the product of the gas density multiplied by thegap width. As the rising voltage is applied to the gap, elec-tron avalanches propagate from the cathode toward the an-ode. The avalanche head is negatively charged, while thepositive ions are left behind remaining “trapped” in the bulkplasma region. Therefore, there is a small chance the positiveions will reach and bombard a polymer surface placed on theanode. The presence of ions in these discharges is limited asthe ionization degree decreases as square of pressure, p,

ne

n0�

1

p2 , �3�

where ne and n0 represent the electron concentration and gasdensity, respectively.

Other plasma reactive species that are capable of modify-ing materials present in the discharge are ultraviolet �UV�photons and ozone. In discharges where the electron densitycan be increased, the UV intensity increases until radiationquenching dominates. An example of this is with increasedpower dissipated in the plasma region. In atmospheric dis-charges the most dominant species besides the electrons arethe oxygen ions, O−, O2

−, O3−, and O2

+. Oxygen is almostalways present in these processes even when the chosen pro-cess gas is other than O2, as most experimental systems areeither operated under atmospheric air or not properly sealed.Their relative concentrations of the oxygen species dependon the gases being employed, the presence of microdis-charges, and the electrical characteristics of the discharge.Ozone generation is the result of the dissociation of molecu-lar oxygen by electron impact. All these reactions occur inthe nanosecond scale up to a few seconds making atmo-spheric plasma processing of materials a potential candidatefor fast speed processes. Some of the electron initiated reac-tions are listed here:

e + O2 → O�3P� + O�3P� + e,

e + O2 → O�3P� + O�1D� + e,

e + H2O → OH + H + e,

e + N2 → N + N + e,

while quenching of the atomic �O�� and molecular excitedspecies �N2

�� is also observed,

O� + H2O → 2OH,

N2� + O2 → N2 + O + O.

In air discharges the presence of nitrogen ions N+, N2+,

atoms, and excited atomic and molecular species adds to thecomplexity of reactions in the gas phase.35–38 In addition toozone, several oxides are generated: NO, N2O, NO2, NO3,and N2O5, which can be detected by emission spectroscopyor by dynamic mass spectrometry.39,40

The main reactions involving charged, neutral, and ex-cited atoms and molecules in the discharge region aresummarized:41

• Formation of negative ions occurs when low energy elec-trons collide with atoms or molecules. Cations are formedthrough the reactions with noble gases and nitrogen com-pounds, while oxygen-based gases often form negativeions. The conductivity of a plasma is strongly influencedby capturing low energy electrons. The field strength formaintaining a stable current is therefore much higher in anelectronegative gas. This explains the impact of eventraces of impurities, such as air and water present in thedischarge medium, which tend to extinguish the plasma. Itshould be noted here that negative ions and metastableshave much longer lifetimes than electrons.

• Recombination of positive ions and electrons or anionsrecreates the initial neutral state that existed prior to thedischarge ignition. It can cause some additional effectssuch as the emission of recombination radiation and theformation of excited states of high chemical activity. Therecombination time of an atmospheric-pressure dischargeis estimated to be in the order of a microsecond.

• Following plasma ignition the active particles are formedin a thin channel ranging from 0.5 to 200 �m in diameter.The filament density, spacing, and overlap are critical if theentire gas volume is studied.

• The high-energy electrons in the microdischarge tip causeionization and excitation to higher electronic states. Themost important example is the N2�C� state because it leadsto the UV emission of the second positive system, extend-ing from 328 to 434 nm. The excitation also leads to highvibrational relaxation by which these states decay throughcollisions to rotational and translational excitation. Emis-sion spectroscopy is often used as a tool to calculate thevibrational and rotational energies.

• Metastable states are also important because they are oftenclose to resonant levels, so a collision with a low energyelectron can cause them to become resonant, lose the en-ergy within �1 �s by emitting a vuv photon, and willreturn to the ground state. It is also possible that they trans-fer all their energy to a different molecule, causing its ion-ization. As the metastable energy state of helium �He��,vastly used as carrier gas for DBDs, is very high �19.8 eV�and as most impurities have ionization potentials lowerthan the threshold of He�, they can be ionized through thePenning ionization.42 For instance, quenching of nitrogenmetastables can cause dissociation of water molecules toform OH radicals in humid air discharges.

• Radicals are formed by electron impact dissociation ofmolecules in the microdischarge head region. The dissocia-tion energy is usually lower than the ionization energywhile radicals are mainly created directly by these colli-sions, e.g., O, H, OH, and N. These may react rapidly withmolecules to form secondary radicals such as HO2 or O3.43

If contaminants such as SO2 or NO are present O, HO2,and O3 can subsequently oxidize them to acids andnitrogen-based radicals can reduce NO to N2.




The key to avoiding or limiting the appearance of fila-ments in atmospheric-pressure discharges is controlling thedischarge parameters to retain a glow discharge versus a fila-mentary discharge. According to the studies performed byGherardi and Massines19 two conditions need to be satisfiedin a N2 DBD to initiate a Townsend breakdown,44 conse-quently leading to a uniform glow, rather than a streamerbreakdown. First, a high initial density of electrons is desiredprior to the electrical breakdown in order to initiate severalsmall avalanches instead of a single strong avalanche.45 Sec-ond, provided that several small avalanches are initiated, theions confined to the primary avalanches need to have enoughtime to reach the cathode before the electrical field becomeslarge enough to produce large avalanches. More informationon the various discharge regimes can be found in Ref. 46.

To simulate the action of a short-lived microdischarge,either a short, nanopulsed, high-voltage pulse is applied or anelectron beam is injected. In both cases, it is necessary toderive the rate coefficients for electron impact collisions inthe gas mixture under consideration by solving the Boltz-mann equation. This requires a reliable set of electron colli-sion cross sections. For DBD studies the local field approxi-mation is normally used, assuming that the electron energydistribution is in equilibrium, where the electric field and allrate coefficients can be calculated as a sole function of themean electron energy or the reduced field E /n.

Modeling microdischarge formation is closely related tocomputing gas breakdown at atmospheric pressure. In theearly phases of discharge development there is little differ-ence seen in the breakdown between metal electrodes and thebreakdown in a gap with one or two dielectric materials cov-ering the electrodes. When an overvoltage is applied to adischarge gap at atmospheric pressure an electron avalanchethat starts at the cathode soon reaches a critical stage wherethe local “eigenfield” caused by space charge accumulationat the avalanche head leads to a state where extremely faststreamer propagation toward both electrodes becomes pos-sible. Breakdown between metal electrodes was studied byMarode et al.47 and by Babaeva and Naidis48 among others.The results of this computational work reveal that extremelyhigh electric fields occur at the microdischarge tip. It alsorevealed that a thin current channel is formed. Propagation iscaused by ionization waves traveling at a speed much higherthan the electron drift velocity.

At atmospheric pressure equilibrium is reached in pico-seconds as the electrons accelerated in the electric field par-ticipate in numerous reactions with the neutral gas species.Appreciable voltage changes and corresponding electric fieldchanges are much slower, typically in the nanosecond range.This justifies the use of stationary solutions of the Boltzmannequation. Excitation and dissociation by electron collisionare extremely fast processes followed by free radical reac-tions that occur at an intermediate time scale, typically1–100 �s at atmospheric pressure. Most free radical reac-tions will therefore be completed before any substantial dis-

placement of the involved species by either diffusion or re-combination can take place. These processes take muchlonger and occur at millisecond time scales.

A review of existing literature leads to the conclusion thatin most cases, the main mechanism responsible for its sur-face modification is the free radical and neutral reactionswhere a material is exposed to a DBD. In simulating theaction of many microdischarges in complicated gas mixturesa first approach neglecting the electron kinetics may involverepetitively injecting certain concentrations of free radicalsand computation of the chemical reactions occurring afterthe injection. This approach is normally taken if reliableelectron collision cross sections are missing for some com-ponents of the gas mixture under consideration.

DBDs are ideal for the modification of soft materials suchas polymers because they are low-temperature environmentsthat will not result in the degradation of the exposed mate-rial. In most gases, the reduced field at breakdown corre-sponds to electron energies of about 1–10 eV. This is theideal range for the excitation of atomic and molecular spe-cies and the scission of chemical bonds, such as CuC,CuH, and CuO that are present in the polymer backbone.Also, as polymers are regarded as dielectric materials, it isimportant for the electric field not to exceed their dielectricstrength values. At atmospheric-pressure electron densities of1014–1015 cm3 and current densities in the range of100–1000 A /cm2 are reached. Typical charges transportedby individual microdischarges are on the order of 100 pC.

In order to optimize the design of a DBD system, there isa necessity to calculate the power consumed in the silentdischarge over a wide range of discharge conditions. In pre-vious studies, Takaki et al. investigated the silent dischargecurrent-voltage characteristics in coaxial wire-cylinder reac-tor aimed for ozone generation as well as reactors with par-allel plate geometry, which are more suitable for materialprocessing.49 In this work, the discharge power consumed ina parallel plate DBD reactor with two dielectric layers wasmeasured as a function of the applied voltage and the dis-charge gap. A simplified model of the reactor’s equivalentelectrical circuit was used to calculate the consumed powerand shows the existence of an optimal discharge gap, whichwas found to be �1.5 mm. At this interelectrode distancethe electric power delivered to the discharge was maximal,which is advantageous for material processing applications.The reactor mean power is often evaluated by the parallel-epiped area formed by the V-Q Lissajous figures, which isdirectly proportional to the consumed energy per onecycle.50,51

Another technique aiming to assist in the characterizationof the plasma phase is OES. The optical emission ofatmospheric-pressure discharges can be analyzed by spectro-scopic techniques. Monochromators have sufficient reso-lution for a discharge at 760 Torr but problems such as thelow intensity and the small gap are often present. A tech-nique that is well suited in this case is the time correlatedsingle-photon counting method. It uses an optical trigger to




determine the timing of the photon to be counted. A time-to-amplitude converter can be included to obtain a time reso-lution down to 0.1 ns.

Although dielectric barrier discharges operating underuniform and filamentary mode have been used for severaldecades, detailed studies of their structure and the character-istics of the microdischarges could not be performed untilsensitive image converters and fast current monitoring tech-niques became available. This requires a synergistic studycombining modeling work and experimentation using state-of-the-art equipment and techniques. All methods mentionedabove can be used for this purpose with detection possibili-ties that have only recently become available, e.g.,generation-IV charge-coupled-device-camera equipment.This opens the door to using newer techniques, such as thecoherent anti-Stokes Raman scattering which is already be-ing tested in barrier discharges52 and mass spectrometricdiagnostics.53 Another possibility is the use of cavity ring-down spectroscopy, which is an extremely sensitive absorp-tion technique that could be used to determine short livingintermediate species.

IV. SURFACE MODIFICATION OF POLYMERS INDIELECTRIC BARRIER DISCHARGESUNDER ATMOSPHERIC PRESSURE

The inherently low surface energy and chemical inertnessof polymeric substrates have generally required elaboratesurface modification schemes to optimize favorable bondinginteractions. Due to the absence of polar groups in the back-bone, polyethylene requires a great degree of surface activa-tion to promote adhesion, which may include chemical treat-ments with maleic anhydride,54 ultraviolet grafting ofacrylates and methacrylates,55 silane coupling agents,56 par-affin wax,57 gamma-ray irradiation treatments, or plasmatreatments.58,59

Plasmas are known60 to have an effect on altering thesurface properties of polymers being exposed to such envi-ronments. The impact of dielectric barrier discharges can besummarized as cleaning and functionalization of the exposedsurfaces through the removal of surface residual impurities,grafting of new functional groups and improving the surfacewettability �Fig. 7�.

These all lead to improved printability, dyeability, andadhesion. This is achieved due to the presence of free radi-cals present in the plasma phase, mainly oxygen containing,which are grafted on the surface during the plasma exposure.To overcome the dominant effect of oxygen species from theresidual air present in the reactor system Borcia et al.61 ex-perimented with pure N2 dielectric barrier discharges. Ac-cording to this work, 8% of atomic nitrogen was detected on

the surface of ultrahigh polyethylene films after 0.5 s oftreatment. Nitrogen-rich polymer surfaces are reported topromote cell adhesion62,63 due to the presence of aminegroups and their positive charge that are capable of attractingnegatively charged proteins, DNA, and cells in aqueous so-lutions at physiological pH values.64–67 In a study comparinglow-pressure and atmospheric-pressure plasmas nitrogen-richpolyethylene films having similar properties weredeposited.68 It was shown that films grown under high pres-sure conditions were rich in unsaturated nitriles and iminegroups, while those deposited under low-pressure containedhigher concentrations of amine groups. The overall charac-terization of the films deposited under atmospheric pressurerevealed the presence of partially oligomeric weakly cross-linked films, which were water soluble and, therefore, unsuit-able for cell culture applications.

In a polymer, the density of grafting sites for graftpolymerization69 is limited by the availability of uOHgroups that serve as anchoring sites for surrogate surfaceinitiators. As the treatments take place under ambient airconditions a mild etching effect is expected to occur on thesurface of the polymer leading to increased microroughness.

Plasma treatment of polymer textiles has attracted a lot ofattention during the past decade due to both the environmen-tal and energy conversion benefits in developing high perfor-mance materials. Exposure to plasmas under atmosphericpressure can improve woven fabric and fiber properties suchas wicking, dyeing, printing, surface adhesion, mechanical,fracture, and ballistic properties which are both time and costefficient.70 Textiles are porous materials and therefore plas-mas tend to penetrate and modify both sides of the materialsimultaneously, requiring treatments of less than 1 s.71

A. Hydrophilic and hydrophobic surfaces

Plasma treatment of polymers under low or atmosphericpressure is a well-known method to change the surfacewettability.72,73 Wettability of solid surfaces is an importantproperty which depends on both the surface free energy andthe surface roughness.74 Special surface geometries such asnanoneedles, nanotubes, nanorods, nanopillars, nanofiber ar-rays, silicon etched structures, and combined microscale/nanoscale surfaces have been used to achieve surfacesuperhydrophobicity.75–79 The effect is dependent on theplasma composition, but in most cases exposure to atmo-spheric plasmas increases both the hydrophilicity and thesurface energy of treated surfaces. Results show that evenafter a short exposure to the discharge, of less than 1 s and upto 1 min, the water contact angle of hydrophobic polyethyl-ene decreases more than 50%, compared to the untreatedsample.80 By increasing the exposure time, the measured wa-ter contact angles continue to drop, reaching a plateau afterseveral seconds, depending on the plasma and polymer beingemployed.

The hydrophilic character of the treated films can be in-terpreted as the result of the chemical modification of thesurface, by the addition of polar groups through the plasmatreatment, and increased surface roughness due to the ap-

FIG. 7. �Color online� Interaction of atmospheric-pressure plasmas with ma-terials, etching, deposition, and surface functionalization.




pearance of microdepressions, which will be described later.The attachment of new polar functional groups is confirmedby the calculation of the total surface energy81–86 and theanalysis of its two components, polar �p and dispersive �d.87

As expected, the control polyethylene film has a very lowpolar surface energy of 0.12�10−3 N /m and a total surfaceenergy �tot of 39.82�10−3 N /m, which is primarily due toits dispersive component. After a 1.3 s exposure to a helium-oxygen DBD, the dispersive component decreases to 32.95�10−3 N /m and a dramatic increase of the polar componentis observed, as it reaches a value of 18.49�10−3 N /m. Pro-longing the treatment time results in an increase of �p, while�d remains approximately the same. Compared to the controlfilm and after 39 s treatment, we observed a significant en-hancement of �p �25.2�10−3 N /m�, accompanied by a 41%increase of the total surface energy �56.22�10−3 N /m�, asshown in Table I.

Several other polymers, besides the results described ear-lier for polyethylene, have been tested with success. Fang etal.88 demonstrated the surface modification of polytetrafluo-roethylene �PTFE� by filamentary and homogeneous dielec-tric discharges in air. Results of this study reveal the im-provement of the surface hydrophilicity of PTFE,demonstrated by the contact angle reduction of 50° after a 40s exposure to air DBDs. Scanning electron microscopy�SEM� analysis revealed changes in the surface morphologyas the plasma treated films are dominated by irregular sur-face protrusions.

The major drawback of plasma-induced hydrophobicity ofpolymers is the aging effect, also known as hydrophobicrecovery.89 This occurs when the polar groups grafted on thepolymer surface by the plasma exposure move from the sur-face to the bulk of the polymer, as depicted in Fig. 8.

The aging process starts immediately after the plasmatreatment and can lead to a 5°–25° increase of the watercontact angle90,91 for polyethylene terephthalate and polya-mide films.

The wettability improvement of nylon fibers was also re-ported in Ref. 80 where the fibers were exposed toatmospheric-pressure plasmas. The study utilized a scaled upprocess by using an industrial type system, designed toplasma treat polymer films and woven fabrics up to 0.5 mwide at an average speed of 5 m/min.

Enhancing the hydrophobicity of polymer surfaces finds alot of applications as protective water and soil repellant andself-cleaning textiles require this property. De Geyter et al.92

reported a 30% increase in the water contact angle of poly-propylene films using a helium-fluorocarbon DBD for 3 s. Inthis study, a significant amount of oxygen �9% atomic con-centration� was present on the polymer surface after theplasma exposure. This proved that the presence of atmo-spheric air and humidity93 is crucial and the incorporation ofoxygen is inevitable. The fluorination of a polymer can beachieved through the deposition of a coating94 by using fluo-rocarbon gases such as tetrafluoromethane,95 tetrafluoroeth-ylene, octafluoropropane, and hexafluoropropylene96 orthrough the functionalization of surfaces with fluorine-containing groups originating in DBDs. Hydrogen can beadded to the fluorocarbon monomer to lower the F/CFx den-sity ratio in the gas phase and favor the deposition of coat-ings with variable chemical composition �i.e., F/C ratio� andcross-linking degree.97 It was demonstrated that CF4uO2

fed DBDs are able to etch a silicon substrate;98 however, theetching process is expected to compete with the depositionprocess and the controlled operation of the DBD will lead tofavoring deposition over the etching process and viceversa.99

B. Surface characterization of polymers treated underDBDs

X-ray photoelectron spectroscopy �XPS� is a very usefulmethod for the characterization of plasma treated materialsbecause it is a surface oriented technique and also due to thefact that plasmas are known to alter only the surface proper-ties of materials exposed to them, while the bulk propertiesremain unaffected. XPS results of polyethylene films treatedunder helium-oxygen DBDs reveal that the modified surfacesexhibit a surface rich in oxygen-containing groups. It sug-gests that the plasma treatment induced the formation of car-boxyl, hydroxyl, and carbonyl groups on the surface. Theseare polar groups that can enhance the hydrophilicity of thepolymer. The oxygen uptake can be attributed to either thegeneration of atomic oxygen during the plasma treatment�resulting from reactions in the bulk plasma area with O2�and/or the reaction of the resulting “activated” surface withatmospheric oxygen. While it is likely that both mechanismscontribute to the resulting functional groups, the exactmechanism cannot be isolated and identified easily.

TABLE I. Dependence of water contact angle and surface energy of UHM-WPE on plasma treatment time.

Treatment time�s�

WCA angle�deg�

�p

�10−3 N /m��d

�10−3 N /m��tot

�10−3 N /m�

Untreated 101.7 0.12 39.82 39.941.3 53.0 18.49 32.95 51.446.5 46.8 23.14 30.25 53.3919.5 42.3 24.70 31.58 56.2839 40.0 25.20 31.02 56.22

0 5 10 15 2040

50

60

70

80

90

100

110

1.3 s6.5 s19.5 s39 s

Watercontactangle(deg)

Timeafter plasma treatement (days)

untreated film

FIG. 8. Aging study of plasma exposed UHMWPE films.




Figure 9�a� represents the XPS spectrum of a pristine ul-trahigh molecular weight polyethylene �UHMWPE� film.Traces of oxygen were detected, possibly the result of mildoxidation during the film processing. Figure 9�b� shows theincrease of oxygen concentration on the polyethylene filmsurface due to the plasma treatment. The films were treatedunder HeuO2, where the oxygen gas flow was 2% of thetotal gas flow, and the treatment times ranged from 7.8 to70.2 s.

After the plasma treatment, the carbon signal from thesurface decreases, giving rise to an increased oxygen signal.We observed a dependence of the oxygen concentration onthe treatment time; the longer the plasma exposure, thegreater the degree of surface oxidation. Results for the filmstreated for 7.8 s in the helium-oxygen plasma showed19.07% oxygen atomic concentration and 80.88% carbon,while the measured atomic concentrations for the films ex-posed to the plasma for 23.4 s were 21.59% and 78.07%,respectively. The components of the C 1s high resolutionspectra were labeled as the following: C1 at 285 eV repre-sents the –CH2 groups, C2 at 286.6 eV assigned to CuO oruCuOH, C3 at 288 eV, and C4 at 289.2 eV correspondingto uCvO and uCOOR or uCOOH, respectively. The C1contribution to the total carbon signal decreased from 97.9%to about 70% after the plasma treatment, indicating the oxi-

dation of the surface through its interaction with the plasmaactive species. The C2 peak corresponding to CuO orCuOH exhibited a fivefold increase after a short exposureof 7.8 s. The C3 peak reached saturation when the substratematerial was treated for times longer than 7.8 s; further treat-ment does not increase the intensity of the peak. Finally, theCOOH bond was enhanced and was 9.4%, 11.2% and 13.2%for the treatment times of 7.8, 23.4, and 70.2 s, respectively.

C. Studies of the surface morphology after plasmatreatment

SEM is often used to investigate the physical effects onthe DBD treated surfaces of the polymer films. The SEMmicrographs shown in Fig. 10 demonstrate the impact onsurface morphology of microdischarges, similar to the Lich-tenberg figures used to footprint the filamentary discharge.The untreated UHMWPE film is fairly smooth, with no spe-cial features, when compared to the film plasma treated un-der HeuO2 for 1.1 and 5.3 s. The latter exhibits the forma-tion of microdepressions having an average size of 5 �m,which is caused by the plasma exposure. The formation ofthese craterlike features is evident even when the sampleswere exposed to the discharge for limited times, as short as 1s, and is dependent on the plasma process parameters and thephysical properties of the polymer being treated.

The formation of the craters seen in Fig. 10�b� cannot besolely attributed to the impact of the streamers. We expect anactivation process and mild surface etching due to plasmaexposure, and atomic oxygen causes etching of polymer sur-faces when present in the discharge. The role of the heliumplasma, especially with the high flow rates used in this work,is to remove any impurities residing on the surface and,through energy transfer mechanisms, to cause chain scissionand the formation of cross-linked layers on the polymer sur-faces. These layers provide stability to the material and act asa barrier to surface changes. Moreover, helium is expected toimpose the Penning ionization to other molecules present—mainly O2. Therefore, the combined action of helium andoxygen is expected to result in cleaning, etching, and activa-

1200 1000 800 600 400 200 00

60012001800240030001200 1000 800 600 400 200 00

300600900120015001800

Counts

BindingEnergy (eV)

(a)

(b)

FIG. 9. XPS analysis of as-received and plasma treated UHMWPE films.

FIG. 10. �Color online� SEM micrographs of plasma treated UHMWPE films showing the formation of microdepressions due to exposure to filamentarydischarges.




tion of the surface. Mild etching and increased microrough-ness was also observed on polypropylene fibers100 afterHeuO2 plasma treatments �Fig. 11�.

From Pappas et al.100 UHMWPE, polyethylene terephtha-late, polyamide �Nylon�, and PTFE films were plasma modi-fied under HeuO2 discharges. While all of the materialsstudied exhibited changes in their surface chemical compo-sition after the plasma exposure, the morphology of the poly-tetrafluoroethylene surfaces was not affected. It was con-cluded that besides the plasma experimental parameters, thephysical properties of the substrate also impact the morphol-ogy of the surfaces. The polymers that were studied havedifferent resistances to UV light, dielectric strength, surfaceresistivity, thermal conductivity, crystallinity, and surface po-larity. According to the technical data provided by the manu-facturer of the polymers, PTFE has two to three times higherdielectric strength than UHMWPE �28 kV/mm� which sug-gests that PTFE can intrinsically sustain higher electricfields, without breaking down, unlike UHMWPE. Also, thesurface resistivity of UHMWPE is 1013 � /sq, while that ofPTFE is 1017 � /sq, an indication that PTFE has strongerresistance to the flow of electric current over its surface.While PTFE displays excellent resistance to UV light, UH-MWPE exhibits poor resistance to it, and helium plasmashave intense emission lines in the UV region. While no cor-relation has yet been drawn between the physical propertiesof the polymers studied and the formation of microdepres-sions, it is postulated that the dielectric properties and sur-face polarizability of the polymers will impact the interactionof the substrate and plasma discharge.

The plasma-induced surface modification also has an im-pact on the adhesive strength of polymers. In a study101 UH-MWPE was modified under several DBD gas chemistries�HeuO2, N2, and air�. Results reveal that the surface oxida-tion can be controlled and is dependent on the experimental

conditions. Short exposure of 1.3 s to the plasmas listedabove results in mild oxidation of the surface and a signifi-cant increase �up to 42%� of the surface energy.

In this work, the plasma treated surfaces appear to berougher and of textured morphology which makes them idealcandidates for composite systems �Fig. 12�, as both promotethe mechanical interlocking and frictional energy dissipationeffects when bonded to another substrate.

Evidence of the improved bond strength was provided byT-peel testing where films treated under helium-oxygen andnitrogen plasma discharges for 1.3 s, which show an en-hancement from 13.7 N/m �as received� to 959.4 N/m and1048.6 N/m, respectively.

FIG. 11. �Color online� SEM micrographs of polypropylene fibers before and after plasma treatment �courtesy of Y. Liang, Drexel University�.

FIG. 12. �Color online� Interaction of plasma functionalized surfaces withpolymer matrices in composite structures.




V. PLASMA ASSISTED DEPOSITION OF ORGANICAND INORGANIC COATINGS

Atmospheric plasma technology has been employed indeposition processes through the delivery and evaporation ofliquid precursors in the plasma phase. This process is knownas atmospheric-pressure plasma liquid deposition �APPLD�and is applicable to large area substrate materials such astextiles �wovens and fibers�, paper, films, and foils. The de-velopment of flexible solar cells and displays, for instance,requires the use of large-scale cost efficient processes thatwill lead to the production of transparent uniform coatingsthat are free of defects. The equipment used for these pro-cesses is usually a roll-to-roll reactor with a parallel plateelectrode DBD configuration.102,103 Also, jet plasmas are of-ten used for processing three-dimensional nonflexible mate-rials.

The direct injection of an aerosol of liquid precursors intoa homogeneous atmospheric-pressure plasma leads to theformation of a thin conformal layer of polymerized coatingonto a substrate surface that is in contact with the plasma.The difference between APPLD and plasma-enhancedchemical vapor deposition �PECVD� lies in the fact that inthe APPLD case the precursor is not vaporized and, there-fore, the plasma is used to functionalize the substrate prior todeposition. The combination of liquid precursor and diffuseatmospheric-pressure plasma ensures that this process retainsall the original functional properties of the liquid precursor—even for large complex molecules. This is a property uniqueto APPLD, as almost all other atmospheric-pressure plasmaprocesses destroy complex precursors.

This enables tailoring of the surface chemistry with a spe-cific chemical functionality and/or a specific surface re-sponse. This type of surface engineering can be applied to avariety of different substrate materials across a wide range ofapplications. Thus, advanced surface properties that includebiofunctionality, oil repellency, and adhesion promotion areavailable from APPLD technology. This allows for the pros-pect of plasma processing expanding to a wide range of newhigh investment industrial applications.

Perhaps, the most commonly deposited coating usingplasmas operating under atmospheric pressure is silicon ox-ide. SiO2 is a material that has garnered a lot of interest inrecent year due to its numerous applications. These includegas barrier films, semiconductor devices, optical devices, andbiomedical materials.104,105 Several precursors have beenemployed, mainly organosilicon compounds, such as tetra-ethoxysilane �TEOS�,106 tetramethoxysilane,107 hexamethyl-disiloxane �HMDSO�, and vinyltriethoxysilane.108,109 Orga-nosilicon precursors are preferred for atmospheric plasmadeposition due to the fact that they are easy to handle andnonexplosive compared to silane-based mixtures, SiH4. Fromthe comparison of atmospheric and low-pressure silicon ox-ide deposition processes, Sawada et al.106 concluded that thedeposition rate and film properties were very similar. Theprecursor flow rate plays a crucial role; as reported by Zhu etal.,110 at low flow rates of 20 SCCM the deposited films aresmooth and continuous while particle formation is observed

above this critical flow with more prolonged deposition.Eventually, the surface features grow larger and cracks startto form. The plasma-enhanced chemical vapor deposition ofsilica films under atmospheric pressure111 is preferred overother methods such as thermal oxidation of silicon and ther-mal and chemical vapor deposition.112–114

In the PECVD case, the liquid precursor is typicallyplaced in a container and kept at constant temperature, whichis dependent on the vapor pressure of the chosen precursor.Vapor is generated by bubbling argon or other inert gases andthen is carried into the plasma chamber. The addition ofother gases such as O2 or H2 results in the growth of analmost carbon-free silica film, where the carbon content isless than 2%. However, the observed deposition rate is lower��120 Å /s� compared to the case107 where only a singlecarrier gas—Ar, He, N2, and N2O—was employed.115 Thepresence of carbon in the films grown under atmosphericpressure is one of the two challenges encountered in theseprocesses, as the resulting materials have a SiOxCy structureinstead of a pure SiO2. The achievement of inorganic versusorganic films depends on the experimental setup and systemgeometry used. For instance, in the work of Raballand etal.116 where a microplasma jet system was employed, at lowHMDSO flow rates of less than 0.1 SCCM, the SiOxHz filmscontain no carbon and exhibit an O/Si ratio close to 2, whilefor higher precursor rates the carbon content in the film isfound to be around 20%. High deposition rates, of 3000Å/min, were recorded when TEOS was used with anatmospheric-pressure plasma jet fed with helium and oxygengases powered with a 13.56 MHz power supply.117 The di-electric properties of the resulting films had a dependency onsubstrate temperature. Those grown under 150 °C had a di-electric constant of 5 while if deposited under higher tem-perature �350 °C� their dielectric constant decreased to 3.81comparable to that obtained by thermal oxidation processesand can be attributed to the lack of uOH groups at higherdeposition temperature.

The deposition of hydrocarbon films under atmosphericpressure has also attracted a lot of interest over the past de-cade. Diamond-like carbon films exhibit high hardness118

and elastic modulus, low friction coefficient, chemical inert-ness, and optical transparency and can be used as protectivecoatings against wear and corrosion.119 Commonly usedgases for their growth include ethylene, acetylene, andCO2.120

The properties of the plasma deposited polymer films arenot determined by the monomer employed as it is involvedin several gas-phase reactions producing radicals, unsatur-ated groups, and low molecular weight fragments. Therefore,unlike commercially available polymers, plasma depositedpolymers are not characterized by repeating units.121

Gallium-doped ZnO films were grown on silicon sub-strates by atmospheric-pressure metal-organic chemical va-por deposition using diethylzinc and water as reactant gasesand triethyl gallium as a n-type dopant gas.122




VI. DIELECTRIC BARRIER DISCHARGES FOR THETREATMENT OF BIOMATERIALS

A. Inactivation of biological agents

According to the existing literature atmospheric plasmascan promote sterilization, bacteria and microorganism inac-tivation, biocompatibility, and even cell growth.123–125 Theplasma impact depends strongly on both the plasma sourceand gases used. This nonthermal plasma sterilization processis dependent on the impact of the charged species, electricfields, reactive neutral species, UV radiation, and heatpresent in the ignited discharge.

In a 2001 article published by Moisan126 it is claimed thatthree plasma-induced mechanisms lead to the inactivation ofbiological agents: �i� deconstruction of the microorganismgenetic material �DNA� by UV radiation produced in theplasma, �ii� an etching process induced by the plasma reac-tive species which leads to erosion of the bacteria and bio-logical organisms and subsequent destruction, and �iii� ero-sion of the microorganisms through intrinsicphotodesorption. The photon-induced desorption is due tothe scission of chemical bonds in the microorganism afterbeing exposed to plasma-generated UV radiation, allowingits atoms to form volatile compounds. A report127 publishedin 2006 lists a few more inactivation causes which can besummarized as destruction of the cytoplasmatic membrane,protein, and DNA material through the oxygen-containingplasma radicals and in the case of Gram-negative bacteria,lysis of the bacterium membrane due to electrostatic forcesimposed by the charged particles present in the plasmaphase. According to others, it is believed128 that in nonther-mal atmospheric-pressure plasmas, no significant UV emis-sion is present, which reduces the UV contribution in theseprocesses. Even when the UV contribution to sterilization isnegligible, the synergy of other species such as radicals andcharged particles129 still plays a dominant role.

Atmospheric-pressure plasmas are proven effective inkilling bacteria, parasites, fungi, spores, and viruses on livingtissue.130–133 Two different approaches have gained popular-ity over the past decade, involving �a� indirect treatment134

through the use of a plasma jet source exposing the treatedsurfaces to the plasma afterglow and �b� direct treatmentwhere the infected surfaces serve as a plasma electrode. Parket al.135 exposed filter paper inoculated with bacteria andfungi to an atmospheric plasma torch which generated an Ardischarge. Scanning electron microscopy images of thetreated filter paper showed that the spores were reduced todebris after a 20 s plasma exposure.

Small area treatments can be performed using a plasmaneedle setup136 seen in Fig. 13, which can kill harmful bac-teria, require a treatment time of tenths of seconds, have areproducible killing effect, and are site specific. They arealso site specific, targeting only the infected area, which isseveral millimeters in diameter.137 Dielectric barrier dis-charges have been applied not only on solid surfaces but alsoin gas mixtures for sterilization of air streams, bioaerosols,138

and water.139 A brief overview of this aspect of plasma ster-ilization is presented later in this paper, in Sec. VII.

Even though the killing efficacy of direct plasmas andplasma afterglows under atmospheric pressure has provensuccessful, there are still questions on the long term effect onthe tissues being exposed to them and the impact on humanDNA.

B. Functionalization and surface patterning usingatmospheric plasmas

Atmospheric plasma processing has the potential to beemployed in patterning processes to modify polymers forbiomedical applications by improving their biocompatibility.The term “biocompatibility” generally refers to the fact that aprosthesis or a medical device is nontoxic, properly performsthe function it was engineered for, and is tolerated by thebiological medium where it is “at work” for a reasonableperiod of time.140 Air plasmas have been employed to im-prove the hydrophilicity of poly�methyl methacrylate�, poly-styrene, and polycarbonate and thus promote the growth ofcarcinoma cells to be studied.141

Conventional methods aiming to pattern biomaterials, tofunctionalize the surface of interest, and to provide struc-tural, chemical, and biological cues with patterns to controlcell morphology and their functions are mainly based onphotolithography, self-assembled monolayers, andstamping.142–145 Through these processes cell adhesive andcell repellant biomolecules form stable covalent bonds withthe biomaterial surface in a controlled pattern. The plasmaalternative method does not require clean room instrumenta-tion, long processing time, or complex chemistry often in-volved in other processes, thus making it both cost and timeefficient. Plasma based approaches have recently gained con-siderable interest due to the versatility and efficiency in bothstructural and chemical functionalizations for eliciting rel-evant biological responses in many biomedicalapplications.146–149 Since most biological fluids are waterbased, the hydrophilic or hydrophobic character of polymersurfaces as biomaterials is critical for biocompatibility andcell growth tests.150

Dr. Sun’s group at Drexel University has developed a pro-totype system which performs plasma patterning and biomol-ecule deposition in a one step printing process. An atmo-

FIG. 13. �Color online� Plasma needle setup.




spheric DBD plasma was successfully employed for theattachment and proliferation of osteoblast cells cultured overplasma activated poly--caprolactone scaffolds.151 Figure 14shows images from this microplasma system, where thenozzle tip diameter is 30 �m and the distance between thetip and the substrate is 4mm when oxygen is gradually addedto the helium discharge causing it to extinguish.

VII. DIELECTRIC BARRIER DISCHARGES FORPOLLUTION AND WASTE CONTROL

Another area that is receiving growing attention andwhere DBDs find application is the destruction of toxic com-pounds and pollution control. After initial work on militarytoxic wastes by Clothiaux et al. in 1984152 and Frasier et al.in 1985153 an increasing number of investigations were de-voted to the decomposition of nitrogen and sulfur containingoxides and of volatile organic compounds such as hydrocar-bons, chlorocarbons, and chlorofluorocarbons in silent dis-charges. Volatile organic compounds �VOCs� emitted in aircan have a serious health impact on humans as they partici-pate in photochemical smog formation reactions.

Contamination of exhaust air with gaseous hydrocarbonsor organic solvent vapor occurs in many industrial processes,in chemical processing, in print and paint shops, in semicon-ductor processing, as well as in soil remediation and watertreatment. The plasma energetic species created in a DBDresult in the formation of radicals and excited atomic andmolecular states, mainly of nitrogen and oxygen. Reactiveradicals such as OH, HO, and HO2 are also produced andwill subsequently react with hazardous compounds to formO3, O2, CO2, and H2O, which are less toxic. In humid gasstreams such as combustion exhaust gases the hydroxyl radi-cal �OH� plays an important role. Hydroxyl groups are easilygenerated and do not impose an environmental hazard; na-ture uses OH to clean the troposphere.154,155 The“Volfilter,”156 a planar version of the OAUGDP, uses stripelectrodes energized by a high-voltage low-frequency rfsource to generate plasma and exposed both sides of a sheetof dielectric air filter material. After the filter material re-moves microorganisms from the air stream, the OAUGDplasma kills the captured microorganisms.

Besides the employment of dielectric barrier discharges todestruct VOCs, the production of ozone associated with theiroperation can treat potable and waste water.157 The setupused for this type of treatment is a cylindrical DBD reactor

consisting of a center discharge electrode surrounded by adielectric barrier, a quartz tube, and an outer metal electrode.A high ac voltage of up to 30 kV is applied to the centerelectrode creating discharge pulses which generate gas-phasefree radicals, such as atomic hydrogen, oxygen, and hydroxylradicals, all of which are capable of destroying pollutants.The destruction mechanism involves the conversion ofVOCs to CO2 and H2O at high destruction removal efficien-cies. The method is a promising candidate against the con-ventionally used as plasma reactors have low energy require-ments when, for instance, compared to incineration. Otheradvantages of this method include operation at ambient pres-sures and temperatures, absence of sorbents, and catalysts allcontributing to the minimization of cost of operation. It alsoinvolves the ability to simultaneously destroy organic �ben-zene, methyl ethyl ketone, toluene, trichloroethylene,etc.�158–161 and inorganic �NO and SO2� pollutants.162

In water purification processes the main reason for install-ing ozonation stages is the reduction of turbidity, the removalof color, bad taste, odor, and heavy elements such as Mg andFe, as well as disinfection. Chlorine was vastly used in thepast for water purification, but its usage in potable watertreatment can be harmful for human health, as it may containtrihalomethanes, which are carcinogen compounds. Themethod utilizes air as the reactant gas, while the ozone dis-solution in the water is performed using special reservoirs.Injection and mixing of ozone with water are achievedthrough nozzles and meshes.

Also, the presence of phenolic compounds in river wateroccasionally disturbs the production of drinking water. Phe-nols often originate in industrial plants or pesticides. A newozone generator163 was used for the removal of phenol andchlorophenols from water. The ozone generator was based oncoaxial dielectric barrier discharge system and operated in airat an atmospheric pressure. Flowing water acted as one ofdielectric layers, while ozone and ozonized water were gen-erated and coexisted in the discharge.

VIII. CONCLUSION

In an article published in 2007, Roth et al.164 stated: “Anyplasma processing task possible with a glow discharge invacuum can also be performed by a glow discharge at oneatmosphere, provided that long mean free paths are not re-quired.” This statement reveals the potential of atmosphericplasmas to be applied in several technological areas.

Industrial plasma engineering utilizing atmosphericequipment has a promising future in materials functionaliza-tion, deposition of organic and inorganic coatings, and ster-ilization of biomaterials and biocidal materials for the fol-lowing reasons:

�i� Unique chemical environments can be obtainedthrough the formation of free electrons, radical, andcharged and excited species;

�ii� Provided a plasma system is available, the processcost and equipment maintenance is very low; and

�iii� Plasma processes performed under atmospheric-pressure conditions are ecofriendly as the formation

FIG. 14. �Color online� Effect of oxygen introduction to a He microplasmafrom left to right oxygen percentage in helium: 0, 1, 2, and 5.




of by-products is minimal, while the produced wasteis insignificant.

In addition to these major industrial, economic, and envi-ronmental advantages, there is also tremendous potential forfuture research as there is still limited understanding of boththe properties of these plasmas and their interactions withmaterials.

ACKNOWLEDGMENTS

The author would like to express her heartfelt gratitude toall the people who helped perform the studies on atmo-spheric plasma processing of materials. She particularlywishes to thank the scientists and engineers at the ArmyResearch Laboratory, Drexel University and University ofBari that have contributed to this work through helpful dis-cussions.

NomenclatureAPGD atmospheric plasma glow discharge;APPLD atmospheric-pressure plasma liquid

deposition;DBD dielectric barrier discharge;OAUGDP one atmosphere uniform glow discharge

plasma;PECVD plasma-enhanced chemical vapor

deposition;PTFE polytetrafluoroethylene;SEM scanning electron microscopy;UHMWPE ultrahigh molecular weight polyethylene;VOC volatile organic compound; andXPS x-ray photoelectron spectroscopy.

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